Structure and method for hyper-abrupt junction varactors

ABSTRACT

Method of fabricating a varactor that includes providing a semiconductor substrate, doping a lower region of the semiconductor substrate with a first dopant at a first energy level, doping a middle region of the semiconductor substrate with a second dopant at a second energy level lower than the first energy level, and doping an upper region of the semiconductor substrate with a third dopant at a third energy level lower than the second energy level.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 10/707,905 filed Jan. 23, 2004, the disclosure of which isexpressly incorporated by reference herein in its entirety.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention generally relates to varactors and, more particularlyrelates to hyper-abrupt (HA) junction varactors and a method offabrication of HA junction varactors in CMOS, rf-CMOS, BiCMOS or analogtechnologies.

2. Background Description

Varactors form a class of tunable semiconductor capacitors typicallyderived from pn-junctions, where the pn-junction is operated in areverse bias state. The capacitance of the varactor may be varied byadjusting the reverse bias voltage, and thus varactors are characterizedby a C-V tuning curve. Varactors are especially useful in oscillatorcircuits, especially voltage-controlled oscillators, where the varactortunability is used to tune the oscillation frequency of the circuit.Thus, varactors find use in cellular phones, televisions and radios,computers, active filters, and wherever a first signal is synchronizedto second signal.

Varactor functioning is most easily understood in the terms of a basiccapacitor. In general, a capacitor consists of two conductive platesseparated by a dielectric. Opposite charges collect on the capacitorplates when a voltage potential is applied across the plates. Thecapacitance of the capacitor, and its ability to hold a certain amountof charge at a certain voltage, depends on the distance between the twoplates, among other parameters. The larger the distance between the twoplates, the less the capacitance and the less charge the capacitor canhold at a given voltage potential.

A pn-junction may function as a capacitor in the reverse bias modebecause the reversed voltage potential causes charge carriers to moveaway from the pn-junction. The facing edges of the p and n regionscollect the charge and act as the conductive plates, i.e. the anode andcathode. As the charge carriers move away from the pn-junction, adepletion region near the junction is formed which is the equivalent ofa dielectric in a standard capacitor. As the voltage potential isincreased, the charge carriers move farther away from the pn-junction,which is equivalent to increasing the distance between the twoconductive plates of a standard capacitor. Accordingly, the capacitanceof a varactor is, in part, voltage dependent and may be tuned over aparticular range. The capacitance of a varactor is also dependent onother parameters such as junction area and the doping densities in thejunction.

The doped region of a varactor is typically formed in semiconductor filmdeposited on the varactor cathode. Different doping profiles within thefilm may be used to achieve different capacitance-voltage tuningrelationships (C-V tuning curves). The first varactors were constrainedto linear doping profiles because of fabrication limitations. Suchvaractors have a C-V tuning curve where capacitance is proportional tothe inverse cube root of the tuning voltage. As fabrication methodsimproved, it became possible to closely control doping profiles, andvaractors with uniform doping profiles became available. The uniformdoped varactors have C-V tuning curves where capacitance is proportionalto the inverse square root of the bias voltage.

For some varactor applications, a linear C-V tuning curve is preferred,and thus HA junction varactors were developed. HA junction varactorshave a doping profile which changes in a controlled non-linear way withdensity of the dopants increasing towards the junction and abruptlydropping to zero at the junction. With a suitable profile, thevaractor's capacitance can be linearly dependant on bias voltage over atleast a portion of the varactor's tuning curve.

HA junction varactors may be made with various methods including ionimplantation and molecular beam epitaxial growth. As noted above, one ofthe parameters which affects the capacitance, and C-V tuning curve, of avaractor is the doping profile within the LTE (low temperatureepitaxial) layer. Thus, as doping density varies, so does the C-V tuningcurve of the varactor. The doping profile may be affected by, amongother things, the thickness of the LTE layer. Consequently, the C-Vtuning curve of varactors in a particular manufacturing batch may varysignificantly from one unit to the next based on variations in the LTElayer thickness. In some examples, the C-V tuning curve has as much as a50% variation in capacitance in the middle of the curve.

The cause of variations of the LTE layer thickness may be due tovariation in initial LTE layer formation as well as changes in the LTElayer thickness caused by subsequent manufacturing steps. Such devicevariation due to manufacturing variation may be difficult for the designengineer to accommodate and lead to complicated circuit designs andextra steps in circuit fabrication.

Though the C-V tunability of varactors may offer the circuit designerincreased freedom in designing certain circuits, known varactors haveC-V tuning curves which may vary substantially from one unit to thenext. Such variations are due to variations in the fabrication process,such as etching, layer formation, and doping the multiple layers ofsemiconductor forming the active region (cathode, collector, junction,and anode) of the varactor. In fact, varactor C-V tuning curves may varyby as much 50% from the nominal specification called for by thedesigner. Accordingly, circuit designs must make more complicatedcircuits to accommodate C-V tuning curve variation. But, suchcomplicated fabrication processes, and circuit complexity increase thecost of varactor implementation.

Referring to FIG. 1, a related art varactor is shown. The varactor 10has a Si (silicon) substrate 12 with an N+ subcollector 14 formedtherein. The N+subcollector 14 is positioned in a lower portion of theSi (silicon) substrate 12 and may be formed by ion implanting methodswell known to those skilled in the art. For example, arsenic ions can beimplanted into the Si substrate 12 to form the subcollector 14 with adosage of about 1.4×10¹⁶ atoms/cm² at about 40 KeV energy levels. Itshould be noted that there are typically diffusion areas adjacent to theN+ subcollector 14, such as, for example, at each end of the N+subcollector 14 where some of the dopants would diffuse into thesurrounding Si substrate 12. The N+ subcollector 14 functions as thecathode of the varactor 10. After forming the N+ subcollector 14 regionin the substrate 12, an optional epitaxial Si layer may be formed atopthe surface of the substrate 12 utilizing conventional epitaxial growingprocesses.

Above a portion of the N+ subcollector 14 is a collector 16. Thecollector 16 is formed by doping the Si substrate 12 with firstconductivity type ions of either N-type or P-type. For example, thecollector 16 may be formed by implanting with phosphorus ions at about6×10¹² atoms/cm² at energy levels of about 700 KeV. Above the N+subcollector 14 on either side of the collector 16 and next to a reachthrough implant 20 are isolation regions 18. The isolation regions 18may be isolation oxides, and may further be shallow trench isolationoxides. In the case where the isolation regions 18 are shallow trenchisolation oxides, the isolation regions 18 may be formed, for example,by conventional lithography, etching, and shallow trench fill methodswell know to those skilled in the art.

At one end of the Si substrate 12 and between two of the isolationregions 18 is the reach-through implant 20. The reach-through implant 20extends from a top surface of the varactor 10 into the Si substrate 12and is in electrical communication with the N+ subcollector 14. Thereach-through implant 20 may be formed using conventional methods wellknown in the art. Accordingly, the same ion dopant utilized to dopeother regions of the varactor 10, such as used for doping the N+subcollector region 14 may be used to form the reach-through implant 20.For example, the reach-through implant 20 may be formed with Sb(antimony) dopant with a 1.4×10¹⁴ atoms/cm² density at about 200 KeV, orit maybe formed with P (phosphorus) dopant with a 4×10¹⁵ atoms/cm²density at about 70 KeV.

On top of the reach-through implant 20 is a silicided region 32. Thesilicided region 32 serves to provide good ohmic contact to theunderlying reach-through implant 20, and may be formed by methods wellknown in the art.

On top of the collector 16 is an HA junction 24. The HA junction 24 isformed in the Si substrate 12 above the collector 16 region. The HAjunction 24 is formed by doping methods well known in the art. Forexample, the HA junction 24 may be formed using an N-type dopant, suchas Sb at a density of 5×10¹² atoms/cm² and an energy of about 40 KeV.

On top of each isolation region 18 are sacrificial layers 30. Thesacrificial layers 30 serve to protect the underlying Si regions duringFET processing. The sacrificial layers 30 are made of, for example, aprotect dielectric such as nitride. The sacrificial layers 30 areinitially deposited across substantially all of the top surface of theSi substrate 12 and protects the surface during later processing. Thedielectric is etched prior to LTE processing and may leave the remainingtopography. The sacrificial layers 30 may be formed, for example by PCVD(plasma chemical vapor deposition).

An LTE layer 26 is deposited over the HA junction 24. The LTE layer 26is made from P-type Si using LTE methods well known to those of ordinaryskill in the art. The LTE layer 26 is later doped with P+ type ions bynecessary bipolar implants or the standard PFET source/drain ionimplant. Accordingly, further bipolar processing steps are requiredafter the HA junction 24 has been formed. Such further bipolarprocessing may alter the thickness, as well as other parameters, of theHA junction causing variation in the final C-V tuning curve of thevaractor.

On top of the LTE region 26 is a silicided layer 34. The silicided layer34 is formed by 1-step or 2-step silicide processes using conductivemetals (i.e.: titanium, cobalt, nickel, etc.) which are well-known inthe art. The silicided layer 34 serves to provide good ohmic contact tothe LTE layer 26.

As noted above, either bipolar processing during and subsequent toforming the related art HA junction causes unit-to-unit manufacturingvariation in related art varactor tuning curves, and such variations invaractors complicates circuit designs. Accordingly, in order to simplifythe design of circuits using varactors, a fabrication process whichproduces varactors having less manufacturing variation in the C-V tuningcurve is desirable. Additionally, HA junction varactors which have lessunit-to-unit manufacturing variation in the C-V tuning curve aredesired.

SUMMARY OF INVENTION

In one aspect of the invention, a varactor having a semiconductormaterial having a continuous column with a lower region, a middleregion, and an upper region is provided. The varactor has a first dopantdisposed in the lower region of the continuous column a second dopantdisposed in the middle region of the continuous column, and a thirddopant disposed in the upper region of the continuous column.

In another aspect, a hyper-abrupt junction varactor having a substratehaving a subcollector region and a plurality of isolation regions isprovided. The hyper-abrupt junction varactor has a first region of afirst conductivity type provided adjacent the subcollector regionbetween at least a pair of the plurality of isolation regions. Thevaractor has a second region of a second conductivity type which isdifferent from the first conductivity type located adjacent the firstregion and between the at least pair of isolation regions.

In still another aspect, the invention includes a method of fabricatinga varactor. The method includes forming a semiconductor substrate, anddoping a lower region of the semiconductor substrate with a firstdopant. The method also includes doping a middle region of thesemiconductor substrate with a second dopant, and doping an upper regionof the semiconductor substrate with a third dopant.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a related art HA junction varactor;

FIG. 2 illustrates a first embodiment of an HA junction varactor inaccordance with the invention;

FIG. 3 illustrates a second embodiment of an HA junction varactor inaccordance with the invention; and

FIG. 4 illustrates a graph of doping profiles of an embodiment of an HAjunction varactor in accordance with the invention.

DETAILED DESCRIPTION

The invention relates to hyper-abrupt (HA) junction varactors and to asimplified method of fabrication of HA junction varactors. The inventionsimplifies varactor fabrication and tightens manufacturing tolerances byeliminating one or more etching or layer formation steps. The varactorformation process avoids etching or layer formation steps by relyingmostly on doping steps to form the active region of the varactor. Thisprocess and resulting structure is less expensive and easier to producewith tighter manufacturing tolerances.

Also, by using the invention, altering the C-V tuning is easilyaccomplished by adjusting the doping rates. Thus, C-V tuning curves areshown to be within design parameters using the invention. This inventionis compatible with complex fabrication processes or complex circuitry.In fact, the design of the invention uses a semiconductor substratewithout little or no etching, or the like, required.

FIG. 2 shows an example of an embodiment in accordance with theinvention. In FIG. 2, varactor 40 is shown having a semiconductormaterial 42. The semiconductor material 42 is doped through severaldoping processes to form an N+ region 44, an HA junction 52, and ananode region 54. The N+ region 44 may also be referred to as an N+subcollector or cathode. These regions may be referred to generally aslower, middle and upper regions, respectively. In one implementation ofthe invention, the N+ region 44 may also include a collector region 50in its upper portion. The collector region 50 may be formed by the tailof the N+ region 44, and thus may include the same dopant as the N+region 44 at a lower dopant density or concentration. These componentsare formed in a continuous column of semiconductor material, generallyrepresented by reference numeral 100.

Referring more specifically to FIG. 2, an embodiment of the HA junctionvaractor 40 in accordance with the invention is shown which reducesdevice variation due to manufacturing variations. The HA junctionvaractor 40 has a Si substrate 42 into which the N+ region 44 isimplanted. The N+ region 44 is implanted using a deep ion implantprocess. For example, the N+ region 44 may be formed using a 1×10¹⁴atom/cm² dose of N-type dopants such as P atoms at about 1 MeV energylevels. Other dopants which may be used include, for example, As(arsenic) and Sb. Accordingly, the N+ region 44 may be formed under arelatively large thickness of Si wafer material, for example about 0.5μm to 1.5 μm, and positioned at a deep level in the Si substrate 42. Assuch, there is no need for a buried subcollector process which involvesimplanting N+ atoms at shallow depth and subsequently growing anepitaxial layer on top of the implanted region. The N+ atomssubsequently diffuse out to form the N+ region 44. Subsequent layers aredeposited on top thereof to hold other dopants. Thus, the N+ region 44may be formed in place within the Si substrate 42, and eliminates theneed to form a buried N+ subcollector.

Either before the Si substrate 42 is doped, or after the N+ region 44 isformed, isolation regions 46 are formed in locations which willultimately be on either side of a collector implant 50 and HA junction52 and reach through implant 48 formed in subsequent steps. Theisolation regions 46 may be shallow trench isolation oxides formed bymethods well known to those of skill in the art. For example, theisolation regions 46 may be formed by etching shallow trenches usingwell know etching techniques, followed by deposition of a trench oxideand CMP (chemical mechanical planarizing).

The N+ region 44 forms the cathode, and the tail of the N+ region dopingprofile forms the collector 50 of the varactor 40. More specifically,the lower energy atoms of the N+ region 44 doping process penetrate to ashallower depth in the Si substrate 42 than the higher energy atoms.These lower energy atoms are referred to as the “tail” of the N+ region44 doping profile and form the dopant occupying the region above the N+region 44, and thus form the collector 50 of the varactor 40. Due to thelow energy levels of the low energy atoms, they do not penetrate intothe Si substrate 42 as far as the higher energy atoms, and thus form aregion above the N+ region 44 of the same dopant atoms as those used toform the N+ region 44.

Accordingly, the collector implant 50 may be formed from N-type dopants,such as, for example, Sb or P atoms. As such, the N+ region 44 and thecollector 50 may be formed in one doping step by taking advantage of theenergy distribution of the atoms in the first doping process.

The HA junction 52 is formed on top of the collector implant 50 betweenthe two isolation regions 46. The HA junction 52 is formed by a dopantimplant into the Si substrate 42 above the collector implant 50. Forexample, the HA junction 52 may be formed with N-type Sb atoms at abouta 1×10¹³ atom/cm² density at about 200 to 300 KeV energy levels, or Asatoms at about a 1×10¹³ atom/cm² density and at about 100-200 KeV energylevels. It should be noted that all or some of the HA junction 52 dopingprofile may overlap a portion of the tail of the deeper N+ region 44dopant profile. Additionally, the HA junction doping profile should bespecifically tailored for the later P-type ion implant profile thatforms the anode.

A source/drain type implant is used to form the anode 54 above the HAimplant 52 between the two isolation regions 46. The anode 54 may beformed with conventional P-type dopants using conventional P+ dopingmethods well known in the art. The anode 54 may be formed, for example,with B (boron) atoms at about a 1×10¹⁵ atom/cm² dosage at about lessthan 15 KeV energy levels. Other dopants which may be used to form theanode 54 include BF.sub.2 (boron fluoride) and In (indium). By utilizinga relatively low energy, the B (boron) atoms are implanted at a shallowdepth near the surface of the Si substrate 42. For example, typicalimplantation depths for the anode 54 range from about 50 Å to about 700Å.

Accordingly, the active portion of the HA junction varactor is a columnof semiconductor material having a lower region with a first dopant, amiddle region with a second dopant, and an upper region with a thirddopant. The first dopant may be an N-type dopant, and may include P, As,and Sb. The second dopant may be an N-type dopant, and may include P,As, and Sb. The third dopant may be a P-type dopant, and may include B,BF.sub.2, In, and Ga (gallium).

Next to one of the isolation regions 46 and above a portion of the N+region 44, a reach-through implant 48 may be formed. The reach-throughimplant 48 extends from the top of the varactor 40 into the Si substrate42 and is in electrical contact or communication with the N+ region 44.The reach-through implant 48 may be formed by conventional methods wellknown in the art. For example, the reach-through implant 48 may beformed by a two step process by doping with Sb atoms at a dosage ofabout 1×10¹⁴ atoms/cm at an energy level of about 200 KeV or P atoms ata dosage of about 1×10⁵ atoms/cm² at an energy level of about 70 KeV.

Still referring to FIG. 2, a first silicided region 56 is formed on thetop surface of the Si substrate 42 above the anode 54. A second silicide58 region is formed on the top of the reach-through implant 48. Thesilicided regions, 56 and 58, provide ohmic contact to the underlyinganode 54 and reach-through implant 48, respectively. The silicidedregions, 56 sand 58, may be simultaneously formed by either a 1-step ora 2-step silicidation process, using, for example, titanium, cobalt,nickel, or other metals as necessary.

Referring to FIG. 3, an example of a second embodiment 60 in accordancewith the invention is shown. In the embodiment shown in FIG. 3, the N+region 44, collector 50, HA junction 52, and anode 54 are formed bysubstantially the same process as the embodiment shown in FIG. 2. Theembodiment of FIG. 3, though, shows a second reach-through implant 59 toa side of the collector 50, which is in electrical communication withthe N+ region 44. The second reach-through implant 59 has a thirdsilicided region 61 formed on a top thereof, and an isolation region 46on either side. The third silicided region 61 is formed using methodssimilar to those which may form the first and second silicided regions,56 and 58.

Consequently, the varactor may be characterized having a substratehaving a subcollector region with adjacent multiple isolation regionswithin the substrate. Also included in the substrate is a first regionof a first conductivity type above the subcollector region and betweenthe isolation regions. The varactor also has a second region of a secondconductivity type which is a different conductivity type that in thefirst region. The second region is located above the first region and isalso between the isolation regions. Additionally, a third dopant regionof the first conductivity type having a dopant concentration lower thana dopant concentration of a first region and a second region isincluded. The third dopant region is located in the substrate between afirst and second dopant regions, and is between the isolation regions.These dopant regions are all formed without the necessity of etchingand/or depositing material.

Referring to FIG. 4, an example of dopant profiles in the active portionof the varactor corresponding to the invention are shown. In the graph,the x-axis represents depth below the surface of the Si substrate, withthe depth increasing to the right of the axis. The y-axis represents thedoping density or concentration, and increases towards the top of theaxis. Deeper penetration into the substrate corresponds to higher energylevels for the dopant.

The N+ region profile 62 represents the doping density as a function ofdepth of the implant resulting from the process which forms the N+region and the collector of the varactor. For example, the deeperportions of the N+ region profile 62 are formed by the higher energyatoms of the 1 MeV energy, with the lower energy atoms forming theshallower regions of the N+ region profile 62. Thus, the left portion ofthe N+ region profile 62 represents the collector region of thevaractor.

As suggested by the N+ region profile 62 of FIG. 4, the N+ region 44 ofFIGS. 2 and 3 may lie on the order of about one μm below the top surfaceof the continuous column of semiconductor substrate 100. Additionally,the dopant concentrations may range, for example, from about 1×10¹⁸atoms/cm³ to about 1×10²⁰ atoms/cm³, depending on the type of dopantused. Also, the dopant dosage densities may form two regions, with alower region having a higher concentration of dopant than an upperregion. In this representation, the upper region of lower dopantconcentration represents the collector region 50, while the lower regionof higher dopant concentration represents the N+ region 44 of thevaractors 40 and 60. The dopant dosage densities may range fromapproximately 1×10¹⁷ atoms/cm³ to approximately 1×10²⁰ atoms/cm³.

The HA junction profile 64 represents the doping density as a functionof depth of the implant resulting from the process which forms the HAjunction 52 of the varactor. As is apparent from the graph, the dopantsforming the HA junction 52 are injected into the Si substrate 42 atenergy levels lower than the energy levels used to dope the N+ region 44and collector 50. Additionally, the density of dopant abruptly tapers tozero as the dopant approaches the surface of the Si substrate 42. Thedepths of the HA junction dopant relative to the N+ region and P+ typedopants are controlled substantially by the relative energy levels ofeach dopant.

Referring again to FIGS. 2 and 3, the HA junction profile 64 indicatesthat the dopant forming the HA junction 52 may lie at a shallower depthin the Si substrate 42 than the N+ region 44 and most of the collector50. The HA junction profile 64 also may occupy a narrower layer of theSi substrate than the N+ region 44. As shown, the HA junction profile 64may overlap a portion of the N+ region profile 44, and more specificallymay overlap a portion of the tail of the N+ region profile 44. Thedopant levels may range from approximately 1×10¹⁷ atoms/cm³ toapproximately 1×10¹⁹ atoms/cm³.

The anode profile 66 of FIG. 4 represents the doping density as afunction of depth of the implant resulting from the anode implantingprocess. The doping profile of the anode profile 66 has a low dopingdensity near the HA junction profile 64, and quickly increases inconcentration towards the surface of the Si substrate. The dopingconcentration then decreases at the surface of the Si substrate;however, the concentration does not fall below the doping concentrationsof the N+ region 62 and HA junction 64 profiles.

Referring again to FIGS. 2 and 3, the anode profile 66 indicates thatthe dopant forming the anode 54 of the varactors, 40 and 60, mostly liesat a shallower depth than the HA junction 52, collector 50 and N+ region44. The anode profile 66 also may overlap a portion of the HA junctionprofile 64 and N+ region profile 62. It should also be noted that theanode profile 66 may extend to or almost to the top surface of the Sisubstrate 42, and that the anode doping may occupy a layer of Sisubstrate 42 of about the same thickness as occupied by the HA junction52. The dopant levels may range from approximately 1×10¹⁹ atoms/cm³ toapproximately 1×10²¹ atoms/cm³.

As thus described, the active region of the varactor including thecathode, collector, HA junction, and anode is formed by three dopingsteps. Each of the three doping steps has approximately less energy thanthe previous doping step in order to deposit its respective dopants atsuccessively shallower depths. Because the active region of the varactoris formed solely by the doping steps, the C-V tuning curve of theresulting varactor is less affected by growing or etching steps, andthere is less manufacturing variation from unit to unit. Thus, theresulting varactor is simpler and less expensive to fabricate, and maybe manufactured to tighter tolerances.

Additionally, the techniques to form the HA junctions of the abovedescribed embodiments may be implemented in a modular form, and thus maybe applied in various types of manufacturing technologies includingCMOS, rf-CMOS and BiCMOS with little or no variation. Also the HAjunction varactor may be manufactured using one additional mask level,although the process is compatible with multiple mask levelmanufacturing processes.

While the invention has been described in terms of embodiments, thoseskilled in the art will recognize that the invention can be practicedwith modification within the spirit and scope of the appended claims.For example, the invention can be readily applicable to bulk substrates.

1. A method of fabricating a varactor, comprising: providing asemiconductor substrate; doping a lower region of the semiconductorsubstrate with a first dopant at a first energy level; doping a middleregion of the semiconductor substrate with a second dopant at a secondenergy level lower than the first energy level; and doping an upperregion of the semiconductor substrate with a third dopant at a thirdenergy level lower than the second energy level.
 2. The method of claim1, further comprising forming a cathode of a varactor in the lowerregion, forming a hyper-abrupt junction in the middle region, andforming an anode in the upper region.
 3. The method of claim 1, furthercomprising selecting the first dopant from a first N-type dopant,selecting the second dopant from a second N-type dopant, and selectingthe third dopant from a P-type dopant.
 4. The method of claim 1, furthercomprising doping a bottom layer of the lower region of a higherconcentration of the first dopant than an upper layer of the lowerregion.
 5. The method of claim 4, further comprising forming a collectorof the varactor in the upper layer of the lower region of thesemiconductor substrate.
 6. The method of claim 1, further comprisingforming at least one isolation region adjacent to the lower, middle, andupper regions of the semiconductor substrate.
 7. The method of claim 1,further comprising forming at least one reach-through implant inelectrical communication with the lower region of the semiconductorsubstrate.
 8. The method of claim 1, further comprising forming asilicide layer on a top of the semiconductor substrate above the upperregion.
 9. The method of claim 1, wherein the second dopant is depositedat a shallower depth than the first dopant and the third dopant isdeposited at a shallower depth than the second.
 10. The method of claim1, wherein only three doping steps are utilized to form the varactorwith a cathode, a collector, an HA junction, and an anode.
 11. A methodof fabricating a varactor, comprising: doping a lower region of asubstrate layer with a first dopant having a dopant profile such thatatoms having a first energy (“A”) penetrate to a first depth (“A′”) inthe substrate layer forming a cathode and atoms having a second energy(“B”) penetrate to a second depth (“B′”) in the substrate layer forminga collector region above the cathode, wherein A>B and A′>B′; doping amiddle region of the substrate layer with a second dopant, the middleregion being tailored for an implant profile that forms an anode, thesecond dopant overlapping the collector region; and doping an upperregion of the substrate layer with a source/drain type implant to formthe anode, wherein the doping of the middle region has approximatelyless energy than the doping of the lower region and the doping of theupper region has approximately less energy than the doping of the middleregion.
 12. The method of claim 11, wherein the forming of the collectorregion and the cathode are formed in a single doping step via energydistribution of a single dopant type.
 13. The method of claim 11,wherein an active portion of the varactor is formed in a column from thesubstrate layer which is a semiconductor material.
 14. A method offabricating a varactor, comprising: forming a semiconductor substrate;doping a lower region of the semiconductor substrate with a first dopantat a first energy level; doping a middle region of the semiconductorsubstrate with a second dopant at a second energy level lower than thefirst energy level; and doping an upper region of the semiconductorsubstrate with a third dopant at a third energy level lower than thesecond energy level, wherein the semiconductor substrate includes acollector region and a cathode that are formed in a single doping stepvia energy distribution of a single dopant type.